Removing unused default switch cases in switches over enums that already account for all enumeration values explicitly.

(This time I believe I've checked all the -Wreturn-type warnings from GCC & added the couple of llvm_unreachables necessary to silence them. If I've missed any, I'll happily fix them as soon as I know about them)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148262 91177308-0d34-0410-b5e6-96231b3b80d8

utils/TableGen/TableGen.cpp

@@ -178,9 +178,6 @@ namespace {
         }
         break;
       }
-      default:
-        assert(1 && "Invalid Action");
-        return true;
       }
   
       return false;